1. Field of the Invention
The present invention relates to a method and apparatus for supplying fuel to internal combustion engines.
2. Description of the prior art
There is known in the prior art a fuel supply apparatus having a fuel supply pump driven by an electric motor, in which, after fuel pressurized by the fuel supply pump is regulated in pressure by means of a pressure regulating valve in relation to the internal pressure of an intake manifold of an internal combustion engine, a required amount of the pressure-regulated fuel is injected into cylinders of the internal combustion engine by the use of fuel injectors. In the above-described conventional fuel supplying apparatus the fuel supply pump always operates at its upper limit speed and the amount of surplus fuel produced by the regulating operation of the pressure regulating valve varies depending upon the engine load. As a result, a large amount of surplus fuel returns through a return passage from the pressure regulating valve to a fuel tank when the engine load is small, whereas a smaller amount of surplus fuel returns by the same route when the engine load is large.
As described above, since the fuel supply pump of the conventional fuel supply system always operates at its upper limit speed, the noise level and energy consumption of the fuel supply pump are large. Furthermore, the surplus fuel returns to the fuel tank through a long return passage provided between the engine and the fuel tank, in most cases causing a large increase in the temperature of fuel. Thus, in the case where the fuel supply pump always operates at its maximum capacity, the surplus fuel quantity cannot be reduced and increase of fuel temperature is promoted. Returning this high temperature fuel to the fuel supply pump may cause the fuel supply pump or the fuel injectors to vapor lock. As a result, the supply of fuel may become impossible.
Moreover, there may be cases where no fuel is supplied from the fuel injectors, as when, for the example, the supply of fuel is cut during deceleration, at the time of engine failure or the like at such times, all of the pressurized fuel supplied from the fuel supply pump will return to the fuel tank. This means that all of the energy used for driving the fuel supply pump is wasted and may result in a pronounced increase in the temperature of the fuel in the fuel tank.